Widespread seafloor methane venting has been reported in many regions of the world’s oceans in the past decade. Yet, the way methane escapes from these deep formations is poorly understood.
More specifically, scientists are puzzled. Throughout the world, several on-site observations have shown vigorous columns of methane gas bubbling up from these formations in some places. The high pressure and low temperature of these deep-sea environments should create a solid frozen layer that would be expected to act as a kind of capstone, preventing gas from escaping.
A new study by MIT scientists could reveal how and why columns of the gas can stream out of the deep formations, known as methane hydrates. Methane hydrate is an ice-like solid that forms from methane–water mixture under elevated-pressure and low-temperature conditions typical of the deep marine settings, often referred to as the hydrate stability zone (HSZ).
Using a combination of deep-sea observations, laboratory experiments, and computer modeling, scientists discovered phenomena that identify and quantify how the gas is released free from the icy grip of a frozen mix of water and methane.
Early on, Xiaojing Fu, now at the University of California at Berkeley- saw photos and videos showing plumes of methane taken from an NOAA research ship in the Gulf of Mexico, revealing the bubble formation process right at the seafloor. It was clear that the bubbles themselves often formed with a frozen crust around them and would float upward with their icy shells like tiny helium balloons.
Later, using sonar, scientists detected similar bubble plumes from a research ship off Virginia’s coast.
Fu said, “This cruise alone detected thousands of these plumes,” says Fu, who led the research project while a graduate student and postdoc at MIT. We could follow these methane bubbles encrusted by hydrate shells into the water column. That’s when we first knew that hydrate forming on these gas interfaces could be a widespread occurrence.”
In any case, precisely what was happening underneath the seafloor to trigger these bubbles’ release stayed obscure. Through a progression of lab experiments and simulations, the mechanisms at work gradually became apparent.
Fu said, “Seismic studies of the subsurface of the seafloor in these vent regions show a series of relatively narrow conduits, or chimneys, through which the gas escapes. But the presence of chunks of gas hydrate from these same formations made it clear that the solid hydrate and the gaseous methane could co-exist.”
To simulate the lab conditions, the analysts used a small two-dimensional setup, sandwiching a gas bubble in a layer of water between two plates of glass under high pressure.
Fu said, “As a gas tries to rise through the seafloor, if it’s forming a hydrate layer when it hits the cold seawater, that should block its progress: It’s running into a wall. So how would that wall not be preventing it from continuous migration?” Using the microfluidic experiments, they found a previously unknown phenomenon at work, which they dubbed crustal fingering.”
“If the gas bubble starts to expand, what we saw is that the expansion of the gas was able to create enough pressure to rupture the hydrate shell essentially. And it’s almost like it’s hatching out of its shell. But instead of each rupture freezing back over with the reforming hydrate, the hydrate formation takes place along the sides of the rising bubble, creating a kind of tube around the bubble as it moves upward.”
“It’s almost like the gas bubble can chisel out its path, and that path is walled by the hydrate solid. This phenomenon they observed at a small scale in the lab, their analysis suggests, is also what would also happen at a much larger scale in the seafloor.”
“That observation was the first time we’ve been aware of a phenomenon like this that could explain how to hydrate formation will not inhibit gas flow, but rather, in this case, it would facilitate it, by providing a conduit and directing the flow. Without that focusing, the flow of gas would be much more diffuse and spread out.”
As the crust of hydrate forms, it hinders more hydrate formation since it forms a barrier between the gas and the seawater. The methane underneath the barrier can, in this manner, endure in its unfrozen, gaseous form for a long time. The combination of these two phenomena— the focusing impact of the hydrate-walled channels and the segregation of the methane gas from the water by a hydrate layer — “goes far toward clarifying why you can have some of this vigorous venting, on account of the hydrate formation, as opposed to being forestalled by it.
Fu said, “A better understanding of the process could help in predicting where and when such methane seeps will be found, and how changes in environmental conditions could affect the distribution and output of these seeps. While there have been suggestions that a warming climate could increase the rate of such venting, there is little evidence of that so far.”
“The temperatures at the depths where these formations occur — 600 meters (1,900 feet) deep or more — are expected to experience a smaller temperature increase than would be needed to trigger a widespread release of the frozen gas.”
Professor Ruben Juanes at MIT said, “Some researchers have suggested that these vast undersea methane formations might someday be harnessed for energy production. However, there would be great technical hurdles to such use. These findings might help in assessing the possibilities.”
Hugh Daigle, an associate professor of petroleum and geosystems engineering at the University of Texas at Austin, said, “The problem of how gas can move through the hydrate stability zone, where we would expect the gas to be immobilized by being converted to hydrate, and instead escape at the seafloor, is still not fully understood. This work presents a probable new mechanism that could plausibly allow this process to occur, and nicely integrates previous laboratory observations with modeling at a larger scale.”
“In a practical sense, the work here takes a phenomenon at a small scale and allows us to use it in a model that only considers larger scales, and will be very useful for implementing in future work.”
- David A. Weitz et al. Crustal fingering facilitates free-gas methane migration through the hydrate stability zone. DOI: 10.1073/pnas.2011064117